1. Field of the Invention
The present invention relates to a new, solvent-free wire enamel composition and to its use.
2. Discussion of Background Information
Polyesterimide enamels are known from the prior art—for example, from DE-A 1445263, DE-A 1495100, and WO 91/07469 (PCT-EP 90/01911). Enamels of these kinds have good mechanical, thermal, and chemical properties. Consequently they have become widespread within the enameled wire industry.
The binders for polyesterimide-containing wire enamels consist of compounds of medium molecular mass, which are virtually insoluble in standard enamel solvents. As solvents for these wire enamels, therefore, cresolic solvents are used. These include phenols, isomers of cresol (monomethylated phenols), and xylenols, i.e., polymethylated phenols (in this regard, cf. L. Wells and H. Strunk, 1993, Proc. Electrical Electronics Insulation Conference, p. 172). The cresolic solvents are typically used together with extenders. Examples thereof that are contemplated include xylene, SolventnaphtaR, toluene, ethylbenzene, cumene, heavy benzene, and also various SolvessoR and ShellsolR products and also DeasolR.
Application to the wire is accomplished by an enameling step and subsequent baking. In the course of this procedure, the resin crosslinks and becomes insoluble and infusible. According to the state of the art, this operation is repeated a number of times (10 to 20 times) until a pore-free film is obtained. The enameling rate is dependent on the wire diameter. The oven temperature is typically between 500 and 700° C. The wire temperature is not more than 300° C. During the enameling procedure, the copper wire is heated and cooled down again a number of times. From an energy standpoint, therefore, this method is costly and complex.
One alternative is to apply binders by an extrusion method. Materials contemplated for such a method include, primarily, thermoplastics. Such materials, however, do not meet the requirements imposed on modern wire enamels and enameled wires, since the heat resistance is typically only 120° C. For modern applications in coil construction and motor construction, however, resistances are required at temperatures of more than 155° C.
The extrusion coating of coiled wires and hence of wire enamels is known from the prior art. For instance, U.S. Pat. No. 4,145,474 describes the extrusion of polyethylene terephthalate. Extrusion coating of wires with polyethersulfone is also known, from DE 2911269 and EP 0017062. EP 0024674 describes the extrusion of polyethylene terephthalate comprising titanium dioxide as a filler. Lastly, EP 0030717 describes the production of a two-layer wire by extrusion.
The stated methods, however, involve pure coating with thermoplastic components. On account of the deficient heat resistance, they are not suitable for modern applications producing heat of more than 155° C. There are no references in any of these specifications to the problem of the deficient heat resistance.
It is now an object of the present invention to provide a solvent-free wire enamel composition which is applied in extrusion methods but nevertheless meets the requirements of the modern applications in coil construction and motor construction. The application is also to meet the requirements, in particular, in temperature ranges of more than 155° C. The resulting enameled wires are to meet the usual standards for coiled wires.